The present invention relates to a type of composite material which includes fiber material as reinforcing material embedded in a mass of matrix metal, and more particularly relates to such a type of composite material in which the reinforcing material is a mixture of alumina fiber material and mineral fiber material and the matrix metal is aluminum, magnesium, copper, zinc, lead, tin, or an alloy having one or more of these as principal component or components.
In the prior art, relatively low melting point metals such as aluminum, magnesium, copper, zinc, lead, tin, or alloys having one or more of these as principal component or components have been very popular for use as materials for members which are in sliding contact with mating members, because of their affinity for such mating members and their good frictional characteristics. However nowadays, along with increasing demands for higher mechanical performance, the conditions in which such materials are required to operate are becoming more and more harsh, and tribological problems such as excessive frictional wear and adhesion burning occur more and more often; in the extreme case, these problems can lead to seizure of a moving member. For instance, if a diesel engine with aluminum alloy pistons is run under extreme conditions, there may arise problems with regard to abnormal wear to the piston ring grooves on the piston, or with regard to burning of the piston and of the piston rings.
One effective means that has been adopted for overcoming these tribological problems has been to reinforce such a relatively low melting point metal or alloy by an admixture of reinforcing fibers made of some extremely hard material. Thus, various composite materials including fibrous materials of various kinds as reinforcing material have been proposed. The advantages of such fiber reinforced materials include improved lightness, improved strength, enhanced wear characteristics, improved resistance to heat and burning, and so on. In particular, such concepts are disclosed in Japanese Patent Laying Open Publications Ser. Nos. Sho 58-93948 (1983), Sho 58-93837 (1983), Sho 58-93841 (1983), and Sho 59-70736(1984), of all of which Japanese patent applications the applicant was the same entity as the assignee of the present patent application, and none of which is it intended hereby to admit as prior art to the present application except insofar as otherwise obliged by law. Further, for the fiber reinforcing material, there have been proposed the following kinds of inorganic fiber materials: alumina fiber material, alumina-silica fiber material, silicon carbide fiber material, silicon nitride fiber material, carbon fiber material, potassium titanate fiber material, and mineral fiber material; and for the matrix metal, aluminum alloy and various other alloys have been suggested. Such prior art composite materials are disclosed, for example, in the above cited Japanese Patent Laying Open Publications Ser. Nos. Sho 58-93837 (1983) and Sho 58-93841 (1983). Of these abovementioned reinforcing fiber materials, for superior wear resistance properties and relatively low cost, the alumina-silica type, that is to say, either alumina fibers or alumina-silica fibers, are preferred--see Japanese Patent Laying Open Publication Ser. No. Sho 58-93837 (1983) and the abovementioned Japanese Patent Laying Open Publication Ser. No. Sho 58-93841 (1983)--and, for extremely low cost, mineral fibers (see Japanese Patent Application Ser. No. Sho 59-219091 (1984)) are preferred. Again, in the case of these various Japanese patent applications, the applicant was the same entity as the assignee of the present patent application, and it is not intended hereby to admit any of them as prior art to the present application except insofar as otherwise obliged by law.
However, in the case of using alumina fibers as the reinforcing material for a composite material, the problem arises that these alumina fibers are very expensive, and hence high cost for the resulting composite material is inevitable. This cost problem, in fact is one of the biggest current obstacles to the practical application of certain composite materials for making many types of actal components. On the other hand, in contrast to the above mentioned alumina fibers, so called alumina-silica fibers whose principal components are alumina and silica are very inexpensive, and have conventionally for example been used in quantity as heat insulation fibers, in which case, particularly in view of their handling characteristics, they are normally used in the amorphous crystalline form; therefore, if such alumina-silica fibers could satisfactorily be used as reinforcing fiber material for a composite material, then the cost could be very much reduced. However, the hardness of such alumina-silica type fibers is substantially less than that of alumina fibers, so that it is easy for the wear resistance of such a composite material to fall short of the optimum. Futher, in the case of using these types of fibers as reinforcing fiber material for a composite material, since alumina-silica fibers, and particularly alumina-silica fibers in the amorphous crystalline phase, are structurally unstable, the problem tends to arise, during manufacture of the composite material, either that the wettability of the reinforcing fibers with respect to the molten matrix metal is poor, or alternatively, when the reinforcing alumina-silica fibers are well wetted by the molten matrix metal, that a reaction between them tends to deteriorate said reinforcing fibers. This can in the worst case so deteriorate the strength of the resulting composite material, due to deterioration of the strength of the fibers themselves, that unacceptable weakness results. This problem particularly tends to occur when the metal used as the matrix metal is one which has a strong tendency to form oxides, such as for example magnesium alloy.
In this connection, hardness in a resulting composite material is also a very desirable characteristic, and in the case that the reinforcing fiber material is relatively expensive alumina fiber material the question arises as to what crystalline structure for the alumina fiber material is desirable. Alumina has various crystalline structure, and the hard crystalline structures include the delta phase, the gamma phase, and the alpha phase. Alumina fibers including these crystalline structures include "Saffil RF" (this is a trademark) alumina fibers made by ICI KK, "Sumitomo" alumina fibers made by Sumitomo Kagaku KK, and "Fiber FP" (this is another trademark) alumina fibers made by the Dupont company, which are 100% alpha alumina. With the use of these types of reinforcing alumina fibers the strength of the composite material becomes very good, but, since these fibers are very hard, if a member made out of composite material including them as reinforcing material is in frictional rubbing contact with a mating member, then the wear amount on the mating member will be increased. On the other hand, a composite material in which the reinforcing fiber material is alumina fibers with a content of from 5% to 60% by weight of alpha alumina fibers, such as are discussed in the above cited Japanese Patent Laying Open Publication Ser. No. Sho 58-93841 (1983), has in itself superior wear resistance, and also has superior frictional characteristics with regard to wear on a mating member, but in the same way as in the caes of composite materials with alumina fibers of the above crystalline structures as reinforcing fibers is expensive as compared to a composite material with alumina-silica fibers as the reinforcing fiber material. It is therefore very difficult to select a crystalline structure of alumina which allows a composite material made from alumina fibers with that structure to be superior in strength and also to be superior in wear resistance, while maintaining a reasonable cost level.
In contrast to the above, so called mineral fibers, of which the principal components are SiO.sub.2, CaO, and Al.sub.2 O.sub.3, are very much less costly than the above mentioned other types of inorganic fibers, and therefore if such mineral fibers are used as reinforcing fibers the cost of the resulting composite material can be very much reduced. Moreover, since such mineral fibers have good wettability with respect to molten matrix metals of the types detailed above, and deleterious reactions with such molten matrix metals are generally slight, therefore, as contrasted with the case in which the reinforcing fibers are fibers which have poor wettability with respect to the molten matrix metal and undergo a deleterious reaction therewith, it is possible to obtain a composite material with excellent mechanical characteristics such as strength. On the other hand, such mineral fibers are inferior to the above mentioned other types of inorganic fiber with regard to strength and hardness, and therefore, as contrasted to the cases where the other types of inorganic fibers mentioned above are utilized, it is difficult to manufacture a composite material using mineral fibers as reinforcing fibers which has excellent strength and wear resistance properties.